In the art of respiration devices, there are well known variety of respiratory masks which cover the nose and/or mouth of a human user in order to provide a continuous seal around the nasal and/or oral areas of the face such that gas may be provided at positive pressure within the mask for consumption by the user. The uses for such masks range from high altitude breathing (i.e., aviation applications) to mining and fire fighting applications, to various medical diagnostic and therapeutic applications.
Where such masks are used in respiratory therapy, in particular treatment of Obstructive Sleep Apnea (OSA) using Continuance Positive Airway Pressure (CPAP) therapy, there is generally provided in the art a vent for washout of the bias flow or expired gases to the atmosphere. Such a vent may be provided for example, as part of the mask, or in the case of some respirators where a further conduit carries the expiratory gases, at the respirator. The washout of gas from the mask is essential to ensure the carbon dioxide build up does not occur over the range of flow rates. In the typical values of pressure in CPAP treatment, usually between 4 cm H2O to 20 cm H2O, prior art attempts at such vents have resulted in patient discomfort due to the effort required to breath out against the positive pressure.